This invention relates generally to the art of helium ionization detectors and more specifically to improvements in helium ionization detection systems.
The helium ionization detector (HID) is one of the most sensitive detectors currently available for gas chromatography. The detector is nonselective, meaning that it is capable of responding to all chromatographable species ranging from the permanent gases to complex organic molecules. Despite its universal response mechanism and high ionization efficiency, the HID detector has not been widely used. The reasons for its limited use include the stringent requirements for high sensitivity operation, instability, and variations in response for selected species as a function of chromatographic conditions. The greatest sensitivity is obtained when ultrapure helium is used as the carrier gas and when contributions from the chromatographic system (e.g., column bleed) to the background current are minimal. Low parts-per-billion concentrations of the permanent gases can be determined under these conditions.
The universal response characteristic which is largely an advantage can also be troublesome since any atmospheric diffusion into the system will reduce the sensitivity of the detector. Long periods of time may be required to stabilize the detector on initial start-up, when changing separation columns, or following any exposure of the system to the atmosphere. The response to the substrate or solvent may also be excessively large requiring long periods between sample analyses to allow the detector to return to initial background conditions. When the detector is over loaded by high concentrations of an analyte, or if the background is high, anomalous peak shapes or polarity inversions may be obtained making it difficult to interpret the results.
Despite these problems, there has been renewed interest in the HID. The characteristic negative response for the permanent gases has been examined and conditions which invert the signals defined. It has also been determined that the detector may be operated in the saturation region of the field intensity with sensitivities comparable to those which may be obtained in the exponential region (i.e., at greater than 350 V). This is due to a decrease in noise level and background current. These reductions, in turn, have allowed gas-liquid partition columns to be used with the detector which extends the applications to include higher molecular weight organics.
Conventionally, the HID has been operated only in a dc mode, wherein a constant polarizing voltage is applied across the cell and the ionization current is measured continuously. In this mode the detector is prone to spontaneous breakdown when high concentrations of an analyte are introduced into the cell. Further, the useful detecting range is rather narrow.
The performance of an HID has been recently improved by operating the detector in a pulsed mode in which the bias voltage applied to the detector is pulsed at a selected frequency and duty cycle to reduce noise and background current levels in the detector's output current response as compared to conventional dc bias operation. This technique is the subject of U.S. patent application for a "Pulsed Helium Ionization Detection System" by Ramsey et al, filed Apr. 9, 1985, presently identified as Ser. No. 721,339 and having a common assignee with the present invention, the subject matter of which is incorporated herein by reference thereto.
However, there is a need for further improvements to extend the analytical capabilities of an HID by extending the upper detection limits while reducing instability and maintaining its inherent sensitivity.